Nucleating and clarifying agents are chemical compositions that may be added to thermoplastic polymers to facilitate formation of the polymer as it changes from molten to solid form in the process of crystallization. Such additives may assist in reducing haze of polymeric structures. Many different chemical compositions are known for this purpose. One major issue in the use of such agents is the amount or degree of clarity that the agent or additive imparts to a finished polymeric article. Reducing haze and thereby increasing clarity of such articles is a constant endeavor in the plastics industry.
In general, the use of nucleating agents is a highly unpredictable technology area. Small or slight changes in a molecular structure can drastically change the ability of a given nucleating composition to nucleate or clarify effectively a polymer composition. There is a large amount of unpredictability in the art of nucleating agents. There are many unknowns regarding the effect of a given substance on polymer morphology during recrystallization of thermoplastics.
As an example of one type of nucleator, dibenzylidene sorbitol (DBS) compounds are common nucleator compounds, particularly for polypropylene end products. Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol (hereinafter DMDBS), available from Milliken and Company of Spartanburg, S.C., USA under the trade name Millad 3988®, provide excellent nucleation characteristics for target polypropylenes and other polyolefins. For example, of great interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, and the like) plastic articles.
Calcium stearate is a very popular acid neutralizer present within typical polypropylene formulations to protect the end product from catalyst residue attack. Unfortunately, many nucleator compounds exhibit undesirable reactions with such compounds within polyolefin articles. For sodium, and other metal ions, it appears that the calcium ion from the stearate transfers positions with the sodium ions of the nucleating agents, rendering the nucleating agents ineffective for their intended function. As a result, such compounds sometimes exhibit unwanted plate-out characteristics and overall reduced nucleation performance (as measured, for example) by a decrease in crystallization temperature during and after polyolefin processing.
Problems that may be encountered with the standard nucleators noted above include inconsistent nucleation due to dispersion problems, resulting in stiffness and impact variation in the polyolefin article. Substantial uniformity in polyolefin production is highly desirable because it results in relatively uniform finished polyolefin articles. If the resultant article does not contain a well-dispersed nucleating agent, the entire article itself may suffer from a lack of rigidity and low impact strength.
Furthermore, storage stability of nucleator compounds and compositions is another potential problem with thermoplastic nucleators. Nucleator compounds are generally provided in powder or granular form to the polyolefin manufacturer. Since uniform small particles of nucleating agent may be imperative to provide the requisite uniform dispersion and performance, such compounds must remain as small particles through storage. Certain nucleators, such as sodium benzoate, exhibit relatively high degrees of hygroscopicity such that the powders made therefrom hydrate easily resulting in particulate agglomeration. Such agglomerated particles may require further milling or other processing for de-agglomeration in order to achieve the desired uniform dispersion within the target thermoplastic. Furthermore, such unwanted agglomeration due to hydration may also cause feeding or handling problems for the user.
Solid bicyclo[2.2.1]heptane dicarboxylate salt-containing thermoplastic nucleating additive formulations are used and sold in the industry. Milliken and Company of Spartanburg, S.C. distributes commercially nucleating agents of such metal salts, under the trade name HYPERFORM®. One such product is known commercially as HPN-68®, which is sold by Milliken and Company. U.S. Pat. Nos. 6,465,551; 6,559,211; 6,521,685; and 6,583,206 relate to such compounds and their use. The dicarboxylate salt is usually provided as a granular formulation, and is known as a very good nucleating agent, particularly for applications that require high crystallization temperatures (Tc).
Also of interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, ethylene copolymer polypropylene, (and the like) plastic articles. As noted previously, calcium stearate compatibility is particularly important. Unfortunately, many nucleators exhibit much deleterious nucleating efficacy with such compounds within polyolefin articles. In order to avoid combinations of these standard nucleators and calcium salts, other nonionic acid neutralizers, such as dihydrotalcite (DHT4-A®), sometimes are necessary for use in conjunction with such nucleators.
Other known compounds useful for nucleation include sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K.K., known commercially as NA-11®), talc, and the like. Such compounds all impart high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications. U.S. Pat. Nos. 4,463,113 and 5,342,868 disclose crystalline synthetic resin compositions of cyclic organophosphoric esters.
The structure that is believed to be used in connection with NA-11® is shown below:

A nucleating agent for polypropylene with a combination of positive material properties, like high Tc, low t1/2, isotropic shrinkage, and high stiffness would be highly desirable. Phosphate ester salts, like NA-11® and NA-21® (manufactured by Asahi Denka Kogyo Kabushiki Kaisha of Japan) are known to incur relatively high stiffness in injection molded articles. However, warpage caused by anisotropic shrinkage is often an undesired side effect of such materials. Such warpage is a disadvantage of using phosphate ester salts, and causes them to be undesirable in many applications.
Thus, it may be seen that each nucleating composition has its advantages and disadvantages. This has created a long-felt need in the polyolefin nucleator compound industry to provide compositions that minimize such problems and provide excellent peak crystallization temperatures for the target polyolefin. Unfortunately, it is a significant challenge to find nucleators exhibiting exceptionally high peak crystallization temperatures, low hygroscopicity, excellent thermal stability, high stiffness, and relatively low amounts of shrinkage or warpage in finished articles. For example, many nucleators cause shrinkage beyond the limits required to keep molded articles within their size specifications. Shrinkage is a significant problem in the industry. The invention disclosed herein is directed at minimizing such problems.
Blends of more than one nucleator have been tried, but are not always successful. Furthermore, this is a highly unpredictable area of the chemical arts, and there is usually no any way of knowing what will work until it is tried, and tested, and a relatively substantial amount of work is done.
U.S. Pat. No. 6,586,007 is directed to a combination of 3,4-dimethylbenzylidene sorbitol (DBS) and p-methylbenzylidene sorbitol (mDBS). U.S. Pat. Nos. 6,521,685 and 6,585,819 are directed to additives that comprise a blend of (a) bicyclic salts, and (b) benzylidene sorbitol acetals.